Defining rules for an expert system
We cannot do homework for students in this network, however I can suggest that several items affecting cost and several usage patterns are missing and the number of rules is shy by an order of magnitude.  I wholeheartedly agree with the educational directives you received.

Consider first developing your lists further to include peripherals like DVD burner, USB devices, and audio.  Whether the user does scientific programming, watches movies on the monitor, develops software, and other specific usage scenarios is also more specific and therefore will produce a better tailored system than the answer to the question of whether the user is a professional.

It is not the metric of the number of rules that is of most importance.  It is the number of operations contained in the rule set that is the guiding metric.  This is because rules in Prolog can be aggregated.  The rough estimate of rule operator count to complete a system is sqrt(i*o)/4, where i is the number of input permutations and o is the number of output permutations.

(This is the application of Shannon Information Theory, that number of bits n = log2 (P'/P), where P' and P are the a posteriorii and a priori probabilities respectively.  The divisor of four is because there are about 16 = 24 operators normally used.)

You may end up with thirty or forty rules.

Create some use cases that exercise the extremes as well as some of the typical cases from among the permutations in inputs and outputs.  Run your system on those cases and observe the system behavior.  Learn how to debug by outputting intermediate results or stepping through rule execution.

There are no shortcuts to researching and developing other than not wasting time worrying about how much time it will take.  You can also optimize your homework time by learning the tools and then stepping back, taking a deep breath, and saying, "I can do this!"